1. Field of the Invention
The present invention relates to an impedance conversion circuit, a video apparatus, an audio apparatus, and a communication apparatus, and can be applied to, for example, a radio receiver, a television receiver, a satellite broadcasting receiver, a video tape recorder, a mobile communication apparatus, and the like. The present invention proposes an impedance conversion circuit which can be used at a frequency higher than that conventionally used and which is suitable for use in an integrated circuit. A driving current is supplied to first and second terminals of the impedance circuit in accordance with voltages of first and second input terminals of the conversion circuit, causing current to flow out of second and first output terminals of the conversion circuit in accordance with first and second terminal voltages of the impedance circuit. A video apparatus using this impedance conversion circuit, and the like are also proposed.
2. Description of the Related Art
Hitherto, in an apparatus, such as an audio apparatus, for processing a signal of a relatively low frequency, impedance based on desired characteristics is produced by an impedance conversion circuit 1 such as that shown in FIG. 5, and an audio signal or the like is processed by this produced impedance.
More specifically, in the impedance conversion circuit 1, five impedance circuits 2 to 6 are connected in series to each other, and operational amplifier circuits 7 and 8 are connected among these impedance circuits 2 to 6. Here, the five impedance circuits 2 to 6 are set at predetermined impedances ZA to ZE in accordance with a required impedance, respectively. The first operational amplifier circuit 7 is connected at its inversion input terminal to the connection mid-point of the second and third impedance circuits 3 and 4 and connected at its non-inversion input terminal to the connection mid-point of the fourth and fifth impedance circuits 5 and 6, and is connected at its output terminal to the connection mid-point of the first and second impedance circuits 2 and 3. The second operational amplifier circuit 8 is connected at its non-inversion input terminal to an input end Vin of the impedance conversion circuit and connected at its inversion input terminal to the connection mid-point of the second and third impedance circuits 3 and 4, and is connected at its output terminal to the connection mid-point of the third and fourth impedance circuits 4 and 5.
This makes it possible for the impedance conversion circuit 1 to express an input impedance Zin viewed from an input end Vin by the following equation, thereby making it possible for the impedance circuits 2 to 6 to be set in various ways in order to set the input impedance Zin to a desired value. ##EQU1##
However, this impedance conversion circuit 1 has a problem in that the operating frequency is limited to 1 MHz or less by the operational amplifier circuits 7 and 8. As a result, it is difficult to apply the impedance conversion circuit 1 to a signal processing circuit in a frequency band, such as a video band.
When the impedance conversion circuit 1 is formed into an integrated circuit in such a manner as to be integrated with another signal processing circuit, the operational amplifier circuits 7 and 8 are formed into an integrated circuit, and the number of elements is increased correspondingly, resulting in the problem of unsuitability for formation into an integrated circuit.